The demand for fire retardant, rigid polyurethane foams has increased sharply in recent years as have the governmental and industry standards that these foams must meet before they may be used in many areas where their use is particularly desired, especially in the construction industry.
Various means are known for imparting thermal stability to polyurethane foams. These include the incorporation of various unreactive fillers and additives, as well as the inclusion in the polyurethane foam of compounds containing functional groups which become chemically bound in the polymeric urethane chain and the coating of rigid polyurethane foam materials with flame retarding materials.
Among the unreactive additives have been inorganic and organic substances. Illustrative of the inorganic additives have been metal oxides such as Sb.sub.2 O.sub.3, ZnO and Al.sub.2 O.sub.3. It has also been suggested heretofore to incorporate a combination of these unreactive additives to secure good fire retardancy.
The use of hydrated alumina and antimony oxide as additives in low density, flexible and semi-flexible polyurethanes incorporating halogen-containing polymers, such as polyvinyl chloride, has also been suggested, illustratively in U.S. Pat. No. 3,810,851. There is also described in U.S. Pat. No. 3,737,400 a polyurethane foam said to possess self-extinguishing characteristics wherein the flame-suppressing agent is ammonium hydroxide hydrate and KCl, K.sub.2 O, KNO.sub.3, Ca(OH).sub.2, Mg(OH).sub.2, K.sub.2 SO.sub.4 and Ba(OH).sub.2.
Further illustrative of the additives for use in rendering polyurethane foams flame retardant and reported heretofore in U.S. Pat. No. 3,262,894 is tris(2-chloroethyl) phosphate in combination with alumina trihydrate.
The improvement in fire retandancy provided by the foregoing additives has often been obtained however at some sacrifice in physical properties. Thus, loadbearing capacity and closed cell content have been found to decrease frequently while moisture vapor pressure transmission often increases. The strength properties of humid aging at elevated temperatures are usually reduced considerably, as well.
To retain the inherent physical properties of the foam considered desirable for use in construction, appliances and the like while imparting an adequate fire retardancy thereto has thus involved a compromise between desired objectives.
One means for alleviating this compromise has been by incorporation of a flame-retarding moiety in the polymer chain itself. Thus, polyurethane foams prepared from the reaction of a polymeric isocyanate; an organic compound incorporating active hydrogen-containing groups reactive with isocyanate moieties; and a dibromobutenediol have, for example, been suggested heretofore. These polyurethanes have been characterized by a significant fire retardancy and have been described broadly for use in fiber, foams and particularly flexible foams, films and the like. It has also been known to prepare rigid foams manifesting a thermal and oxidative stability considered adequate in various applications utilizing thermal insulation by inclusion in the cross-linked urethane polymer of conventional isocyanurates resulting from isocyanate trimerization in the production of the rigid foam.
U.S. Pat. No. 4,094,869 has further suggested a particular rigid polyurethane foam system incorporating 2,3-dibromo-2-butene-1,4-diol along with antimony oxide and hydrated alumina.
While all of the approaches to the thermal stability problem mentioned above have been successful in varying degrees, none have been completely satisfactory in terms both of ability to meet the various industry and governmental standards desired and the desired ease of manufacture.